Gyro platform arrangement



arch ,196? CARL-ERIK GRANQVlST ma ma GYRO PLATFORM ARRANGEMENT Filed Dec. 25, 1963 5 Sheets-Sheet 1 INVENT OR CARL- ERIK GRANQVIST BY QQQ) M ATTORNEYS 14, 1967 CARL-ERIK GRANQVIST 3,303,670

sync PLATFORM ARRANGEMENT Filed Dec. 23, 1963 v 3 Sheets-Sheet a FIG 7 INVENTOR CARL-ERIK GRANQVIST BY 256 Q 0gb? ATTORNEYS March 14, 1967 Filed Dec. 25, 1963 3 Sheets-Sheet 5 1 5 6 4e 67 ea 3? 57 o ff 58 5 3| 5O N i1 7 45 was FIG; 8 64 65 INVENTOR CARL-ERIK GRANQVIST BY $250 Q ATTORNEYS seeking or heading-indication.

United States Patent 3,308,670 GYIIO PLATFORM ARRANGEMENT Carl-Erik Granqvist, Lidingo, Sweden, assignor to AGA Aktiebolag, a corporation of Sweden Filed Dec. 23, 1963, Ser. No. 332,815 Claims priority, application Sweden, Jan. 11, 1963, 304/63 Claims. (Cl. 74-534) The present invention relates to an arrangement in a gyro platform for navigational purposes such as north- The gyro platform is assumed to be journalled in gimbals in such a way that the space orientation of the platform remains substantially unchanged.

The object of the invention is to construct a platform of this type which allows north-seeking and heading-indication with a high degree of accuracy, at the same time, as thegyro platform is of simple construction, which leads to increased dependability with regard to its function and the information to be obtained from it as well as to reduced servicing.

According to the present invention the gyro platform comprises at least three similar gyros rotating at equal speed and mounted so as to respond to a rotation of the platform about an axis of arbitrary orientation relative to the platform. Furthermore, the platform with the gyros is arranged to rotate about an axis which is perpendicular to the platform at a substantially constant speed. The rotation about the lastmentioned axis can be obtained through precession by having each of the gyros subject to a resilient force applied at right angles to the desired precessional rotation of the gyro.

The invention will be described below with reference to the attached drawings wherein:

FIGURE 1 is a schematic drawing illustrating certain portions of a gyroscope and illustrating the principles of the invention.

FIGURE 2 i a plan view of the elements shown in FIG. 1.

FIGURE 3 is a schematic view similar to FIG. 1 but showing two gyros mounted in the gimbals.

FIGURE 4 is a schematic view showing a plurality of gyros mounted on a platform.

FIGURES 5 and 6 are perspective schematic views illustrating the gyros and platform of FIG. 4 with four and three gyros respectively.

FIGURE 7 shows, partially in perspective and partially schematic, one embodiment of the invention.

FIGURE 8 shows, partially in perspective and partially in schematic an additional embodiment of the invention.

In FIG. 1, there is designated by 1 a gyro casing containing a gyro rotor not shown on the drawing which is assumed to rotate at angular speed w about the axis 2. The gyro casing 1 is rotatably journalled by means of an axis of rotation 3 in a gimbal frame 4. This frame in its turn is assumed to be rotatable about an axis 5 journalled in bearings 6 and 7. Furthermore the gyro casing 1 is under the influence of a spring 8 mounted between the gyro casing and the gimbal frame 4 and tending to rotate the casing about the axis 3. This turning moment, in the position of the arrangement shown in FIG. 1, operates counter-clockwise and owing to the gyro effect there results a precession of the gyro, that is, the gyro rotates about the axis 5 with a speed which is proportional to the moment of inertia L, of the gyro rotor and its angular velocity ta as well as the sine of the angle or shown in the drawing to be the angle between the axes of rotation 2 and 5. The speed of rotation of the gimbal frame 4, being represented by the angular velocity on, may be of the order of half a revolution per minute and this speed of rotation will remain extremely constant.

3,363,63 Patented Mar. 14, 19.67

The position of the gyro casing 1 relative to the gimbal frame 4 is indicated by means of a position-indicating sensing member 9 comprising in the embodiment shown an armature 1i) fixedly joined to the gyro casing 1 and which, depending on the position of the casing assumes different positions relative to an E-shaped core 11 whose outer legs are provided with windings to which an A.C. voltage is applied via a terminal 12. The two windings of the outer legs are assumed to be arranged in such a way that the resulting field in the middle leg is zero when the armature 1G is in a position of symmetry relative to the core. Furthermore, the middle leg has a winding in which a voltage is induced, the direction and magnitude of which depends on the position of the armature 10 relative to the core 11. In the position shown in the drawing there is thus no voltage induced in the middle winding.

The middle winding is connected with a winding 13 connected with the rotor of a sine-cosine splitter 14 schematically indicated in the drawing and the rotor of which is fixedly joined to the shaft 5 and the stator of which can be joined to the bearings 6 and 7 as indicated by the connection 15. The stator is furthermore provided with a winding 16 which is connected with a terminal 17.

In FIG. 2, which shows details of the arrangement in plan view, the gyro casing 1 is illustrated with its shaft 2 and the gimbal 4 rotating about the shaft 5. 18 designates a fixed direction in space with which the axis of rotation 3 coincides in the position illustrated in FIGS. 1 and 2. As shown in FIG. 2, B designates the angle between this fixed direction 18 in space and the plane through the axis 5 containing the axis of rotation 2.

Assuming that the gimbal frame 4 is not exposed to any outward turning moment owing to friction in the bearings 6 and 7 or in any other way, the angle at will remain constant as will also the angular velocity m. This velocity is determined by the known relationship M =w 'I -w 'sin (X where M designates the turning moment produced by the spring 8 about the axis 3 and a designates, as before, the angular velocity of the rotation about the axis 5. I is the moment of inertia of the rotor, w its angular velocity and a the angle between the axes 2 and 5.

If the system is exposed to an outward turning moment about the axis 19, which is in the position shown in FIG. 2 forms the angle 8 with the direction 18, the deflection a is changed and from the middle winding in the position-sensing member 9 there is obtained a voltage which is proportional to v wylywycos Olg'sil'l [3 This voltage ,is applied to the winding 13 connected with the rotor of the sine-cosine splitter. This induces in the winding 16 a voltage which is supplied to the terminal 17 and which is proportional to w -I -w -cos a -sin p (3) assuming that the winding 16 is in the same vertical plane containing the direction 18.

FIG. 3 shows an arrangement comprising two gyros whose gimbal frames 4 and 21 are fixedly joined together to enable rotation about a common axis of rotation 5. The'upper gyro corresponds entirely to the gyro according to FIGS. 1 and 2 and the lower gyro is constructed similarly but with an angular displacement of about the axis 5 relative to the upper gyro. The lower gyro casing 22 is rotatable about an axis of rotation 23 and, similarly to the upper gyro, is subjected to a spring 24 mounted between the gyro casing and the gimbal frame 21. From a position-indicating member 25 analogous to member 29 and to which an AC. voltage is applied via a terminal 26, there is taken off in the same manner as from the member 9 a voltage which, assuming a rotation about the axis 27 at angular velocity 40 will be proportional to Through an addition of this voltage to the voltage induced from the winding 13 according to the expression (3), there is obtained at the terminal 17 a voltage proportional to w "I 'w 'COS a If the sine-cosine splitter 14 is joined to a second winding 29 connected with the stator and positioned in a vertical plane at right angles to the vertical plane through the winding 16, it is clear that from this winding as well as from the winding 16 there are obtained voltages which are proportional to a pair of mutually perpendicular axes in the horizontal plane, indicated as the axes 27 and 30 of FIG. 3. The system can then be exposed to a moment tending to cause a rotation with an angular velocity (.9 about the axis 27 and simultaneously to a second moment tending to cause rotation at angular velocity 04 about the axis 36. As is apparent from the above, there can then be taken off from the winding 16 a voltage proportional to the angular velocity o and from the winding 29 a voltage proportional to the angular velocity (0 It was assumed above that there is no friction in the bearings 6 and 7. Friction in these hearings will cause a moment about the axis tending to change the angle a. To enable separation of voltages caused by a moment about the axis 5 from a moment or an angular velocity about either of axes 27 and 30, an arrangement comprising a complementary gyro'may be used as shown in FIG. 4.

According to FIG. 4, there are arranged on a platform 31 two gyro casings 1 and 32, each comprising a gyro rotor not shown on the drawing. Each gyro casing is rotatably suspended in a frame 4 and 33 about axes 3 and 34. Also, each gyro is provided with position-indicating means 9 and 35 which have supplied to them AC. voltage via terminals 12 and 36. The platform 31 is rotatable about a shaft 5 journallcd in hearings 6 and 7. 7

As in the FIGS. 1 and 3 arrangements, the gyro casings are subjected to springs not shown on the drawing and causing turning moments about the axes 3 and 34; The gyro casings are normally in the position shown in FIG. 4, in which the axes of rotation of the gyro rotors form the angle a with the axis 5. Furthermore, the directions of the rotations of the gyro rotors are such as to make them cooperate to impart by precession a rotating movement to the platform 31 about the axis 5. This means that a frictional moment in the bearings 6 and 7 has the same influence on both gyro casings with regard to the angle or, causing it to tend to diminish as a result of the frictional moment. To compensate for the frictional moment there is therefore on the shaft 5 a torque motor 37 having applied to it via amplifiers and an adding circuit 38 voltages from the two position-indicating members 9 and 35. In this manner, the frictional moment can be compensated for so as to render the angular velocity m practically constant independently of the friction.

If there is a turning moment about an axis 27 in a plane at right angles to the axis 5 and resulting in an angular velocity o the angle a will increase for one gyro and decrease by the same amount for the other gyro. A voltage which merely indicates this angular velocity (:1 can therefore be obtained by subtraction of the voltage of one of the position-indicating members from the voltage of the other position-indicating member. This has been achieved in the FIG. 4 embodiment by applying the output voltages from the two position-indicating members 9 and to the winding 13 connected to the rotor of a sine-cosine splitter 14, from which an output voltage is induced in a winding 16 connected with the stator and is applied to a terminal 17.

An additional pair of gyros may be arranged with a 90 mutual displacement around the circumference of the platform 31 and have their position-indicating members connected to a second winding of the rotor of the sine-cosine splitter 14. This makes it possible to obtain in the same manner as in the arrangement according to FIG. 3 an output voltage from the terminal 17 which does not vary with variation of the angle B during the rotation about the axis 5. If the sine-cosine splitter 14 is then provided with a second winding on the stator corresponding to the winding 29 of FIG. 3, there may also be obtained a voltage indicating rotation at an angular velocity 0 about an axis which is perpendicular both to the axis 5 and to the axis 27.

FIG. 5 shows schematically how the various gyros are arranged on the platform 31 for the case that two pairs of complementary gyros are connected with the platform. In this figure, other details, such as position-indicating members, a possible torque motor on the shaft 5 or the sine-cosine splitter, have not been shown, but it is assumed that these are arranged in the manner described above.

The arrangement with four gyros according to FIG. 5 may be thought of as a two-phase system having a mid tapping, which implies that it is possible to substitute therefor a three-phase system in which the platform 31 carries the three gyros 1, 39 and 40 arranged thereon with a mutual angular displacement of 120.

FIG. 7 shows in more detail an arrangement of three gyros 1, 39 and 46 mounted in this manner on the platform 31. The position-indicating member cooperating with each gyro is also shown in the figure and designated 9, 41 and 42. The voltages obtained from these members are added in an amplifier and adding circuit 43 and are then applied to a torque motor 37 tending to maintain constant the angular velocity o of rotation about th axis 5.

A gimbal mounting is provided for the platform 31, the bearings 6 and 7 being connected with a gimbal frame 4-4 which is suspended by means of a shaft 45 and hearing 46 in an external gimbal frame 47. This frame, one half of which is shown in FIG. 7, can be suspended in its turn by means of a shaft 48 in an additional gimbal frame or in a fixed member 49 provided on a vehicle.

The position-sensing members 9, 41 and 42 are further connected each with a winding 50 of the rotor so as to form a synchro 51, the rotor of which is connected with the shaft 5 and whose stator carries two mutually perpendicular windings 52 and 53 and is assumed to be fixedly joined with the gimbal 44 as indicated by the connection 54. From the windings 52 and'53 there can then be obtained voltages representing the magnitude of the angular velocities m and (v during a possible rotation of the platform 31 about one of the axes 45 and 48-. In the FIG. 7 arrangement these voltages are therefore applied via amplifiers 55 and 56 to torquers 57 and 58 which produce in dependence upon the polarity and amplitude of the voltages induced in the windings 52 and 53 a torque about one of the axes 45 and 48 in such a manner as to maintain the space orientation of the axis 5 substantially constant and independent of possible changes in'position of the member 49 carrying the arrangement. This results in a platform rotating about the axis 5 at very constant angular velocity m and whose direction in space represented by the direction of the axis 5 can be maintained substantially constant. This arrangement can be employed for northseeking, as indicated in FIG. 7. To this purpose, each one of windings S2 and 53 is directly connected with a corresponding one of two mutually perpendicular windings 5 9 and 60 of a further sine-cosine splitter 61. The rotor of this splitter carries a winding 62 which can be rotated by means of a motor 63, the position of the winding 62 relative to the stator windings 59 and 60 being indicated by a pointer 64 cooperating with a scale 65 having indications corresponding to north, east, south and west. The winding 62, one end of which is assumed to be grounded, feeds a phase detector and amplifier 66 which delivers an output voltage of polarity and amplitude depending on the angle of rotation of the winding 62 relative to the windings 59 and 60. It is apparent that a voltage will be applied to the motor 63 of such polarity and amplitude as to make the motor 63 tend to turn the winding 62 in such a direction as to reduce to zero the voltage induced therein.

In the north-seeking process, the gimbal frame 44 is held vertical by means of a pendulum system of a kind known per se and not shown on the drawing. It is also assumed that a member 49 carrying the platform with its gimbal system is stationary relative to the earths surface. This means that the platform 31 with its gyros 1, 39 and will be exposed to the influence of the earths rotation, the angular velocity to; of which is supposed to have a direction forming the angle to with the axis 48 of the gimbal system. Consequently, there will be induced in the windings 52 and 53 voltages proportional to w -sin (p and w -cos respectively. When these voltages are applied to corresponding ones of the windings 59 and 66 of the sine-cosine splitter 61, a field is set up therein in a direction relative to one of these windings representing the angle (,0. It is clear, therefore, that, once the winding 62 has been adjusted by the motor 63 in the manner indicated above, the pointer 64, which is assumed to be connected with the shaft of the motor 63 for rotating the winding 62, will indicate the angle (,0. If it is then assumed that the axis 48 is parallel to the lengthwise direction of the vehicle in which the arrangement is mounted, there is obtained in this manner an indication of the direction of the vehicle relative to north.

Assuming that the gimbal frame 44 could be held exactly vertical not only when the vehicle on which the arrangement is mounted is stationary but also when it is in motion, an arrangement according to FIG. 7 could also be used for heading-indication, since there would be obtained on the instrument 64, 65 a continual indication of the angle to between the lengthwise direction of the vehicle represented by the axis 48 and the direction of the earths rotation to However, as was mentioned above, the gimbal frame 44 is held vertical with the aid of a pendulum system, which implies that the arrangement will also respond to accelerations imparted to the vehicle. Suchaccelerations Will cause a rotation about one or each of the axes and 48, thereby inducing voltages in the windings 52 and 53, so that no indication of north is now possible.

An embodiment of the invention which makes possible north-seeking as well as heading-indication is shown in FIG. 8. This embodiment too has three gyros 1, 39 and 40 connected by a platform 31 which is rotatable about the shaft 5. This is journalled in bearings 6 and 7 and may furthermore be carried by a bottom bearing 37, to be described in more detail below. The rotation of the platform 31 is obtained by precession in the manner already described. By means of the torque motor 37 mounted on the shaft 5 and supplied from the amplifier and adding circuit 43, compensation is obtained of the influence of friction in the bearings 6, 7 and 87. In the FIG. 8 embodiment also appear, as already described, the gimbal frame 44 and 47, shafts 45 and 48, synchro 51 with its windings 50, 52 and 53, amplifiers 55 and 56, torquers S7 and 58, the sine-cosine splitter 61 with windings 59, and 62, the motor 63 for rotating the winding 62, the indicating instrument 64, 65 and the detector 66. Furthermire, FIG. 8 indicates the pendulum system serving to maintain the gimbal frame 44 vertical during the north-seeking process. This system comprises a pair of pendulums 67 and 68 connected with the gimbal frame 44 and cooperating with sensing windings 69 and 70.

Each of these windings are similar to the position-sensing members 9, 41 or 42 wherein the pendulums 67 and 68 are armatures and the voltages induced in windings 69 and 70 depend on the position of the armature pendulms 67 and 68 respectively. These voltages induced in wind ings 70 and 69 are applied to the torquers 57 and 53 respectively so that the torquers will maintain the vertical position of the gimbal frame 44.

In order to make it possible to employ the FIG. 8 arrangement for heading-indication, the platform 31 is provided with means for creating an A.C. voltage whose frequency and phase depend on the rotational speed of the platform. This A.C. voltage is adapted to be compared with a reference oscillator A.C. voltage of known frequency and phase, whereby the comparison results in a heading-indication. The A.C. voltage referred to may have a frequency of 3 c.p.s. and may be produced by means of markings provided on the circumference of the platform 31, one for each degree of the circumference, the markings being adapted to be scanned either optically or magnetically by means of a scanning member 71. If it is asumed that the platform 31 has obtained through the precession referred to above a speed of rotation of one revolution in two minutes, this causes the scanning member 71 to produce an A.C. voltage of frequency 3 c.p.s. which is adapted to be applied to a phase detector 72. The detector has a second input circuit connected to a winding 74 which in its turn is connected with the rotor of a sine-cosine splitter 73. There is induced in the winding 74 a voltage whose phase depends on the voltages of a pair of coils 75 and 76 connected with the stator and to which are supplied voltages from a frequency divider 77 with a mutual phase displacement of 'To the frequency divider is applied A.C. voltage from a reference oscillator 78, which may be adapted to yield an A.C. voltage of a frequency of the order of Kc.p.s. This voltage is transformed by the frequency divider 77 to an A.C. voltage of frequency 3 c.p.s. for supplying the two windings 75 and 76 with voltages of mutually perpendicular phase. The voltage created in the phase detector 72, which is obtained by phase comparison between the A.C. voltages from the sensing member 71 and the Winding 74, is applied through an amplifier 79 to the motor 63, the arrangement being such that the motor 63 obtains a voltage of such polarity and amplitude as to make it tend to turn the winding 74 in such a direction thatthere is phase equality between the two A.C. voltages applied to the phase detector 72.

As was mentioned above, the platform 31 will rotate with extremely constant speed. If it is furthermore assumed that the scanning member 71 is connected to follow the heading of the vehicle in which the arrangement is mounted, for instance by being joined with the gimbal frame 44, it follows that a change in course of the vehicle will result primarily in a phase change of the A.C. voltage produced in the scanning member 71. This causes a corresponding rotation of the winding 74, which may be read off on the pointer 64 of the instrument 64, 65, which is connected to the winding via the shaft of the motor 63.

FIG. 8 shows a somewhat modified form of the northseeking arrangement. The voltage obtained from the detector 66 is applied through a smoothing network 80 to the reference oscillator 78, in which it is utilized for controlling the frequency of the reference: oscillator. This may be achieved in the manner indicated in the drawing by having the DC. voltage from the detector 66 control a condenser of the varicap type which is variable according to the applied voltage, and which forms part of a tuning circuit for the reference oscillator 78. This type of arrangement is known per se and is only schematically indicated in the drawing.

If the gimbal system and the winding 62 according to FIG. 8 are in such positions that a voltage is induced in winding 62, the detector 66 will create a DC. voltage depending upon the phase of the applied voltage which changes the frequency of the reference oscillator in such a way that a control voltage is applied via frequency divider 77, sine-cosine splitter 73, phase detector 72 and amplifier 79 to the motor 63. The latter will rotate the winding 62 into a position such that no voltage is induced in the winding. When this has been done, the northseeking process is completed and the connection between the winding 62 and the smoothing network 80 can be broken by means of switch 81.

As mentioned above, the friction in the bearings 6, 7 and 87 causes a torque on the shaft which influences the accuracy of the rotation of the platform 31. This influence may be diminished by making the bearings rotate in the manner shown in connection with the hearing 87. This is arranged to be rotated by a motor 82 via a gearing 83 so as to make the bearing 87 rotate at substantially the same speed as the shaft 5. In the embodiment shown, this is achieved by letting an AC. voltage derived from the scanning member 71 and amplitied in an amplifier 84 drive the motor 82. Since the frequency of the output voltage of the scanning member 71 corresponds directly to the speed of rotation of the platform 31, the bearings 87 will rotate at a speed corresponding to the speed of rotation of the shaft 5. The other bearings 6 and 7 can be made to rotate in principle in the same manner.

It is apparent from the above description that the accurate speed of rotation of the platform about its vertical axis is utilized for course-indicating purposes and that the sensitivity of the platform to rotation about any one of the cross axes is utilized for maintaining attitude and north-seeking. Furthermore, the utilization of rotating gyro systems has great advantages in diminishing errors owing to friction, unbalance and cross couplings.

What is claimed is:

1. A gyro platform for navigational purposes, such as for north-seeking or head-indication, comprising; gimbal frames, a platform mounted in the said gimbal frames, said platform having a shaft of rotation substantially perpendicular to the plane of the platform and connected to the gimbal frames, at least three mutually similar gyros, each gyro having a gyro rotor rotating about its spin axis at a speed equal to that of the other gyros, pivot mountings for each said gyro on the platform and positioned at equiangular distances about said shaft of rotation of the platform, each pivot mounting comprising a pivot axis being substantially parallel to the plane of the platform, a constant torque means on each gyro for seeking to rotate the gyro about said pivot axis thereby generating a constant precessional force acting on the platform, thereby causing a rotation of the platform at substantially constant speed about said shaft of rotation, position-indieating means connected between the platform and each gyro for generating signals indicating changes from a prescribed position of angular equilibrium of the position of each gyro with regard to the platform.

2. A gyro platform as claimed in claim 1 and further comprising signal adding means having an input circuit connected with said position-indicating members and an output circuit, a torquer connected to the shaft of rotation of the platform and being supplied with the output signal from said output circuit for maintaining constant speed of rotation of the platform.

3. A gyro platform as claimed in claim 2 wherein said platform is installed in a travelling vehicle and further comprising means for producing an A.C. voltage of a frequency corresponding to the speed of rotation of the platform with respect to one of the gimbals carrying the platform, a reference oscillator for producing a reference AC. voltage of fixed frequency, and means for comparing the phase of said two A.C. voltages for producing a signal indicating a change in direction of the travelling vehicle in which the platform is installed.

4. A gyro platform as claimed in claim 1 and further comprising signal subtracting means including a sinecosine splitter having first windings connected with the platform and second windings connected With one gimbal frame carrying the platform, and means for supplying said signals from said positions-indicating members to said first windings, shafts for suspending the said gimbals, a gimbal torquer acting to turn each said shaft, means for supplying the output signal from said second windings to said gimbal torquers for maintaining the space orientation of said platform.

5. A gyro platform as claimed in claim 4 and further comprising north-indicating means for indicating a predetermined direction and means for supplying said output signal from said second windings to said north-indicating means.

References Cited by the Examiner UNITED STATES PATENTS 9/1959 Sch'oeppel 74 5.34 4/ 1961 Erdley 74-5 

1. A GYRO PLATFORM FOR NAVIGATIONAL PURPOSES, SUCH AS FOR NORTH-SEEKING OR HEAD-INDICATION, COMPRISING; GIMBAL FRAMES, A PLATFORM MOUNTED IN THE SAID GIMBAL FRAMES, SAID PLATFORM HAVING A SHAFT OF ROTATION SUBSTANTIALLY PERPENDICULAR TO THE PLANE OF THE PLATFORM AND CONNECTED TO THE GIMBAL FRAMES, AT LEAST THREE MUTUALLY SIMILAR GYROS, EACH GYRO HAVING A GYRO ROTOR ROTATING ABOUT ITS SPIN AXIS AT A SPEED EQUAL TO THAT OF THE OTHER GYROS, PIVOT MOUNTINGS FOR EACH SAID GYRO ON THE PLATFORM AND POSITIONED AT EQUIANGULAR DISTANCES ABOUT SAID SHAFT OF ROTATION OF THE PLATFORM, EACH PIVOT MOUNTING COMPRISING A PIVOT AXIS BEING SUBSTANTIALLY PARALLEL TO THE PLANE OF THE PLATFORM, A CONSTANT TORQUE MEANS ON EACH GYRO FOR SEEKING TO ROTATE THE GYRO ABOUT SAID PIVOT AXIS THEREBY GENERATING A CONSTANT PRECESSIONAL FORCE ACTING ON THE PLATFORM, THEREBY CAUSING A ROTATION OF THE PLATFORM AT SUBSTANTIALLY CONSTANT SPEED ABOUT SAID SHAFT OF ROTATION, POSITION-INDICATING MEANS CONNECTED BETWEEN THE PLATFORM AND EACH GYRO FOR GENERATING SIGNALS INDICATING CHANGES FROM A PRESCRIBED POSITION OF ANGULAR EQUILIBRIUM OF THE POSITION OF EACH GYRO WITH REGARD TO THE PLATFORM. 